Continuous countercurrent crystallization process



July 31, 1956 R. P. CAHN 2,757,126

CONTINUOUS COUNTERCURRENT CRYSTALLIZATION PROCESS Filed June 30, 1953LIQUID "NH RECYCLE IOVlll6 BED of III PARTICLES ROBERT F. CAHN INVENTORBY'U). Q 7 ATTORNEY 2,757,126 Patented July 31, 1956 'CONTINUOUSCOUNTERCURRENT CRYSTAL- LIZATION PROCESS Elizabeth, N. J., assignor toEsso Regmeermg Company, a corporation of Robert P. Cahn, search and EDelaware Application June 30, 1953, Serial No. 365,187 4 Claims. (Cl.196-18) The present invention is concerned with an improved process forthe crystallization of constituents and for the removal of the formedcrystals from the mixture. The invention is particularly directed towardan improved continuous dewaxing process wherein wax crystals arecontinuously formed and removed from a waxy distillate containing thesame. In accordance with the present process the dewaxing operation isconducted with the aid of heavy solids, preferably having a low meltingpoint, which solids countercurrently contact the upflowing distillate.

In the refining of hydrocarbon oils such as petroleum oils, it is knownto segregate paraflin waxes from so-called parafiin distillates, waxylubes and the like. The segregation of these waxes is secured by anumber of processes. For example, it is known to chill the selected waxcontaining fraction in order to secure crystallization of the wax and toremove thewax crystals from the oil by filtering, centrifuging and thelike. use various dewaxing solvents such as liquid normally gaseoushydrocarbons, such as propane, as well as other solvents, such asmethylethyl ketone and the like. It is also known to utilize in dewaxingoperations solvent mixtures wherein one solvent comprises a waxprecipitating solvent while the other comprises a solvent having a highsolubility for oil. A solvent mixture of this character, for example,comprises 40% by volume of toluene and 60% by volume of methylethylketone.

The wax segregated from the hydrocarbon oil, usually termed, slack wax,contains from about 10% to 40% of oil. The slack wax is refined usuallyby conventional sweating to produce crude scale wax in a manner toreduce the oil content to less than about 5% by weight. The slack waxmay be distilled to obtain the desired boiling range wax prior tosweating, if desired. This crude scale wax generally has an oil contentof about 2% to 3% by weight. In order to remove this oil from the crudescale wax to produce a refined wax having an oil content below about .5usually below about 3%, various procedures have been proposed andemployed.

While these conventional batch processes are entirely satisfactory theequipment required is relatively expensive. Furthermore, the preferredoperating conditions of these processes are diflicult to control. Theprocesses also require expensive refrigeration, filtering and heatexchange equipment. In accordance with the present process, thesedisadvantages are overcome by utilizing a countercurrent crystallizationtower wherein a liquid is introduced at the top of the tower. Thisliquid crystallizes as it countercurrently contacts up-fiowing liquid.The process of the present invention may be more fully understood byreference to the drawing illustrating one embodiment of the same.

Referring specifically to the drawing, a heated feed oil, which for thepurpose of description is assumed to be a waxy distillate, is introducedinto tower by means of feed line 1. If desired some solvent may beintroduced by means of line 25. This oil is cooled in cooling zone 2 toa temperature in the range from about 75 F. to 150 It is also known toF., preferably to a temperature in the range from 85 F. to 100 F.Aliquid substance, as for example, magnesium nitrate hexahydrate isintroduced into the top of countercurrent treating zone 10. The amountof magnesium nitrate hexahydrate added is in the range from about 40 to200 volumes of magnesium nitrate hexahydrate per 100 volumes of waxyfeed distillate. Preferred concentrations are in the range from about 50to 100 volumes per 100 volumes of waxy feed. The magnesium nitratehexahydrate is injected through appropriate spray nozzles 4 into the oilphase which is kept below the melting point of magnesium nitratehexahydrate by a water coolant or equivalent pumparound stream. Thispumparound stream is cooled in cooling zone 5 and returned to the top ofthe tower by means of line 6. The temperature at the top of the tower isin the range of about F. to 200 F.,

preferably in the range from about F. to F. The magnesium nitratehexahydrate will thus solidify at the top of the tower into smallparticles which settle easily out of the liquid phase .due to the widedensity difference and will begin descending downwardly through the oilphase. As these particles settle, they will countercurrently contactprogressively cooler liquid rising through the column. In accordancewith the present process the coldest point is reached at themid-pumparound level. At this point upflowing liquid is withdrawn fromtower 10 by means of line 7, passed through a refrigeration zone 8 andreturned to the system by means of line 9. The temperature at which theoil is removed is about 0 F. to 40 F., and the temperature at which theoil is returned is about 10 F. to 50 F.

Since very little, if any, wax is present in the upflowing liquid atthis point, exchanger fouling due to the wax will not occur. On theother hand, autorefrigeration may be used in the pumparound section if adiluent such as propane is used. As the particles flow downwardly in thearea below the pumparound section, the magnesium nitrate hexahydrateparticles act as a coolant and crystallization nuclei for the feedmaterial entering the column as illustrated. In the section between thepumparound section and the feed point, the magnesium nitrate hexahydrateparticles absorb heat from the liquid and the solidified wax. The liquidfeed is cooled from the feed temperature to the pumparound temperature.The feed temperature is in the range from about 75 F. to 150 F., whilethe pumparound temperature is in the range from about 0 to 50 F.Furthermore, the wax crystallizes out on the magnesium nitratehexahydrate particles. In this'zone the wax undergoes a certain amountof sweating between the pumparound level and the feed level where theconcentration of wax in the liquid corresponds to the feed composition.

In the area of the column below the point of feed introduction, it ispreferred to introduce a solvent by means In accordance with the presentinvention the magnesium,

nitrate hexahydrate crystals and the wax pass into a heating zone 12heated by heating coils 13 or equivalent means. This area is maintainedat a temperature preferably in the range of about 210 F. to 250 F. Underthese conditions the magnesium nitrate hexahydrate and wax liquefy andform two phases. The Wax phase plus solvent is removed by means of line14, while the liquid magnesium nitrate hexahydrate phase is removed bymeans of line 3 and preferably recycled to the top of the tower. The waxphase is passed to a distillation unit 15 wherein temperature andpressure conditions are adjusted to segregate the solvent which isremoved overhead by means of line 11 and preferably recycled to thesystem. The wax phaseis removedby-means of-line;16 and further handledand refined as desired. The dewaxed oil is removed by means of line 20and passed to distillation zone 21. Temperature and pressure conditions1 in zone '21 are "adapted to remove-over'headby means ofline '22 thesolvent and to segregate by-means of line 23 the dewaxed oil.

The present inventionemploys a solid carrier to act as a nucleus for thewax. in order to avoid filtration and eliminate solid "handling, thissolid is preferably a low melting inorganic salt which is immisciblewith the organic substance processed in the unit.

Satisfactory salts are for example:

Spec. '1\

A particularly desirable salt comprises -magnesium nitrate hexahydrate,the important properties ,of which are as follows:

Formula Mg. ,(NO3)2.6H20 Melting point 100 C.

Heat of fusion 38.2-'cal./g. Specific heat 0.8'87-cal./g./ "C. Density1.464 g./cc.

Its reasonably high density and very high specific heat, low -heat.offusionand .a melting point in the neighborhood of wax, make it wellsuited for the particular application.

While the invention has been described with respect to a dewaxingoperation, it is to be understood that it may be applied to othercrystallization processes andseparations by ureacornplex formationandthe like.

What is claimed is:

-l.'-Improved crystallization process which comprises introducing ,afeedliquid containing a constituent-recoverable by crystallization at anintermediate point of ,a vertical countercurrent treating tower,introducing .into the top .ofsaid .tower, in a plurality of smallstreams, .a liquid law melting inorganic salt immiscible withthe feedwhich has a greater specific gravity in its liquid andsolid phases thanthat of said feed liquid and which has a freezingpoint lower than thetemperature at which said constituent of said feed liquid crystallizes,maintaining the top of said tower at a temperature below the freezingpoint of said salt, whereby said salt is frozen to "small solidparticles which settle downwardly through said tower countercurrent toand in contact with the upflowing liquid feed in an intermediate portionof said tower, whereby said feed liquid is cooled and said constituentcrystallizes on said solid particles, withdrawing the solid particles onwhich said constituent has been crystallized from the bottom of saidtower, heating said particles to a temperature at which two immiscibleliquid phases form, separating the respective liquid phases andrecovering and recycling said liquid salt to the top of said tower.

2. Improved process for the removal'of waxy constituents from a waxyfeed oil which comprises introducing said waxy oil at an intermediatepoint of a vertical countercurrent treating tower, introducing into thetop of said tower, in a plurality of fine streams, a liquid low meltinginorganic salt immiscible with the feed which has a greater specificgravity in its liquid and solid forms than said waxy oil and which has afreezing point lower than the temperature at which said waxyconstituents are crystallized from said waxy oil, maintaining the top ofsaid tower at a temperature below the melting point of said salt,whereby said salt is frozen to small solid particles which settledownwardly through said tower countercurrent to and in contact with theupflowing waxy oil in an intermediate portion of said tower,crystallizing substantially all of said waxy constituents of said oilonto said solid particles in said intermediate zone, withdrawing dewaxedoil from said tower, cooling said dewaxed oil 'to a temperature belowthe freezing point of said salt outside of said tower, re-introducingthe cool dewaxed oil into the top of said tower, whereby said liquidsalt is solidified, Withdrawing solid particles on which wax has beencrystallized from the bottom of said tower, heating said particles abovethe melting point of said wax to form separate phases of wax and liquidsalt, separating the respective phases, and recycling said liquid saltto the top of said tower.

3. Process as defined by claim 2 wherein the salt comprises magnesiumnitrate hexahydrate, wherein the temperature of the waxy oil is'in therange from about to 150 F. and wherein the temperature at the top of thetower is in the range from about to 200 F.

4. Process vas defined by claim 3 wherein propane is introduced intosaid tower at a point below the point of injection of the feed stream.

ReferencesCitedin the file :of this patent UNITED STATES PATENTS1,779,287 Pfafi et val. Oct. 21, 1930 2,062,355 Ellis Dec. 1, 19362,141,361 Pilat et al Dec. 27, 1938 2,322,438 Henryet .al June 22, 19432,622,114 Carney Dec. 16, 1952

1. IMPROVED CRYSTALLIZATION PROCESS WHICH COMPRISES INTRODUCING A FEEDLIQUID CONTAINING A CONSTITUENT RECOVERABLE BY CRYSTALLIZATION AT ANINTERMEDIATE POINT OF A VERTICAL COUNTERCURRENT TREATING TOWER,INTRODUCING INTO THE TOP OF SAID TOWER, IN A PLURALITY OF SMALL STREAMS,A LIQUID LAW MELTING INORGANIC SALT IMMISCIBLE WITH THE FEED WHICH HAS AGREATER SPECIFIC GRAVITY IN ITS LIQUID AND SOLID PHASES THAN THAT OFSAID FEED LIQUID AND WHICH HAS A FREEZING POINT LOWER THAN THETEMPERATURE AT WHICH HAS SAID CONSTITUENT OF SAID FEED LIQUIDCRYSTALLIZES, MAINTAINING THE TOP OF SAID TOWER AT A TEMPERATURE BELOWTHE FREEZING POINT OF SAID SALT, WHEREBY SAID SALT IS FROZEN TO SMALLSOLID PARTICLES WHICH SETTLE DOWNWARDLY THROUGH SAID TOWERCOUNTERCURRENT TO AND IN CONTACT WITH THE UPFLOWING LIQUID FEED IN ANINTERMEDIATE PORTION OF SAID TOWER, WHEREBY SAID FEED LIQUID IS COOLEDAND SAID CONSTITUENT CRYSTALLIZES ON SAID SOLID PARTICLES, WITHDRAWINGTHE SOLID PARTICLES ON WHICH SAID CONSTITUENT HAS BEEN CRYSTALLIZED FROMTHE BOTTOM OF SAID TOWER, HEATING SAID PARTICLES TO A TEMPERATURE ATWHICH TWO IMMISCIBLE LIQUID PHASES FORM, SEPARATING THE RESPECTIVELIQUID PHASES AND RECOVERING AND RECYCLING SAID LIQUID SALT TO THE TOPOF SAID TOWER.